It has previously been known to produce mineral fiber acoustical boards by incorporating a small amount of expanded perlite (usually 25% or less by weight) in a mineral fiber slurry and depositing the slurry on a moving wire screen of a Fourdrinier or cylinder machine. Such boards commonly contain a high weight percentage of mineral fiber, usually at least 50%. The expanded perlite is primarily used as a bulking agent to reduce the density of the mineral fiber boards. In such applications, the perlite is expanded to a very low density, usually between 2-6 pounds per cubic foot, and the size of the perlite particles ranges between U.S. Sieve Nos. 20 and 200. The presence of a high proportion of mineral fiber is used to provide interconnected void spaces, and an overall porous board structure.
A thermal insulation board is also a type of perlite board made by the Fourdrinier or cylinder process. Such a thermal insulation board generally comprises by weight 60-80% of expanded perlite, 20-30% of cellulosic fibers, and 3-10% of a sizing or binding agent such as asphalt. The expanded perlite is again a low density product ranging between 2-6 lbs./cu.ft. and having a particle size range between U.S. Sieve Nos. 20 and 200. Since the thermal conductivity of this perlite insulation board generally decreases as the board density decreases, the board is produced at the lowest density possible consistent with adequate board strength. Such boards are commonly produced in a density range of 10-12 lbs./cu.ft. to meet minimum board strength requirements. The thermal insulation board when formed and compressed to the desired thickness, exhibits a tight, low porosity structure that imparts good thermal insulating properties, but poor sound absorbing properties.